The present invention relates to a method and apparatus for producing a substrate having a micro circuit pattern for a semiconductor device, a liquid crystal, or the like, and particularly relates to a technique for inspecting a pattern for a semiconductor device or a photomask, that is, the present invention relates to a technique for inspecting a pattern on a wafer in a way of semiconductor device producing process and a technique for performing comparison and inspection by using an electron beam.
Inspection of a semiconductor wafer will be described as an example.
A semiconductor device is produced by repeating a process of transferring, by lithographing and etching, a pattern formed in a photomask onto a semiconductor wafer. In a semiconductor device producing process, the state of lithographing, etching, or the like, generation of particles, and so on, exert a large influence on the yield of the semiconductor device. Accordingly, in order to detect occurrence of abnormality or failure in an early stage or preparatorily, conventionally, a method of inspecting a pattern on a semiconductor wafer is carried out in a way of producing process.
As for a method of inspecting a defect existing in a pattern on a semiconductor wafer, a defect inspecting apparatus in which white light is irradiated onto a semiconductor wafer so that circuit patterns of the same kind in a plurality of LSIs are compared with each other by using an optical image, has been put into practice. The outline of the inspecting method has been described in “Monthly Semiconductor World”, August issue, pp. 96-99, 1995. Further, as a inspecting method using an optical image, a method in which an optically illuminated region on a substrate is formed as an image by means of a time-delay integrating sensor so that the characteristic of the image is compared with designed characteristic inputted in advance to thereby detect a defect, has been disclosed in JP-A-3-167456 or a method in which the deterioration of an image at the time of acquisition of the image is monitored so that the deterioration of the image is corrected at the time of detection of the image to thereby perform comparison and inspection in a stabler optical image, has been disclosed in JP-B-6-58220. If a semiconductor wafer in a way of producing process was inspected by such an optical inspection method, the pattern residue or defect having a light-transmissible silicon oxide film or a photoresist material on its surface could not be detected. Further, an etching remainder or a incomplete-open failure in a micro conduct hole smaller than the resolution of an optical system could not be detected. Further, a defect generated in a wiring-pattern stepped bottom portion could not be detected.
As described above, with the advance of reduction in size of the circuit pattern and complication in shape of the circuit pattern and with the advance of diversification of the material, it has become difficult to detect a defect by using an optical image. Therefore, a method for comparing and inspecting a circuit pattern by using an electron beam image having higher resolution than that of the optical image has been proposed. When a circuit pattern is compared and inspected by means of an electron beam image, in order to obtain a practical inspection time, the image needs to be acquired at a very high speed in comparison with observation by using a scanning electron microscopy (hereinafter abbreviated to SEM). Further, it is necessary to secure resolution and an SN ratio in the image acquired at a high speed.
As a pattern comparison and inspection apparatus using an electron beam, a method in which an electron beam with an electron-beam current not smaller than 100 times (10 nA) as large as the current in the general SEM is irradiated onto an electrically conductive substrate (such as an X-ray mask, or the like) to detect any electrons among secondary electrons, reflected electrons and transmitted electrons generated therefrom and compare/inspect an image formed from a signal of the electrons to thereby automatically detect a defect is disclosed in J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991), J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992), JP-A-5-258703 and U.S. Pat. No. 5,502,306.
Further, as a method for inspecting or observing a circuit substrate having an insulating material by means of an electron beam, a method in which a stabler image is acquired by irradiation of a low-accelerated electron beam not higher than 2 keV in order to reduce the influence of charge has been disclosed in JP-A-59-155941 and “Electron and Ion Beams Handbook” (THE NIKKAN KOGYO SHINBUN, Ltd.), pp. 622-623. Further, a method in which ions are irradiated from the back of a semiconductor substrate has been disclosed in JP-A-2-15546 and a method in which light is irradiated onto a surface of a semiconductor substrate to thereby cancel charge of an insulating material is disclosed in JP-A-6-338280.
Further, in a large-current and low-accelerated electron beam, it is difficult to acquire a high-resolution image because of a space-charge effect. As a measure to solve this problem, a method in which a high-accelerated electron beam is retarded just before a sample so that a substantially low-accelerated electron beam is irradiated onto the sample is disclosed in JP-A-5-258703.
As a method for acquiring an electron-beam image at a high speed, a method in which an image is acquired by continuously irradiating an electron beam onto a semiconductor wafer on a sample stage while continuously moving the sample stage is disclosed in JP-A-59-160948 and JP-A-5-258703. Further, a structure constituted by a scintillator (Al-vapor deposited fluorescent material), a light guide and a photo-multiplier is used as a secondary electron detecting apparatus used conventionally in the SEM. A detecting apparatus of this type is, however, poor in frequency responsibility because light emission from the fluorescent material is detected, so that the detecting apparatus of this type is unsuitable for formation of an electron beam image at a high speed. As a detecting apparatus for detecting a high-frequency secondary electron signal to solve this problem, a detection means using a semiconductor detector is disclosed in JP-A-5-258703.
When a circuit pattern in a process for producing a micro-structure semiconductor device was detected by using the aforementioned prior art optical inspection method, it was possible to detect the residue of a silicon oxide film, a resist material, or the like, which was formed from an optically transmissible material and which was sufficiently short in the optical distance depending on the optical wavelength and refractive index used for inspection, and it was difficult to detect an etching remainder or a incomplete-open failure in a micro conduct hole which was linear so that the width of a short side thereof was not larger than the resolution of an optical system.
On the other hand, in the observation and inspection using the SEM, there are two problems as follows. One problem is that a very long time is required for inspecting a circuit pattern on the whole surface of a semiconductor wafer because the conventional method, by means of the SEM, for forming an electron-beam image needs a very long time. Accordingly, in order to obtain practical throughput in a semiconductor device producing process, or the like, it was necessary to acquire an electron-beam image at a very high speed. It was further necessary to secure the SN ratio of the electron-beam image acquired at a high speed and to keep accuracy in a predetermined value.
The other problem was that it was difficult to obtain a stable contrast image in inspection by means of an electron beam and to obtain a predetermined value of inspection accuracy in the case where the material constituting a circuit pattern as a subject to be inspected was formed from an electrically insulating material such as a resist, a silicon oxide film, or the like, or in the case where the material was formed from a mixture of an electrically insulating material and an electrically conducting material. This is because, when an electron beam is irradiated onto a matter, secondary electrons are generated from the irradiated portion of the material but the matter is charged because the irradiated current value is not equal to the secondary electron current value in the case where the subject to be inspected is an electrically insulating material. When charge occurs, efficiency in generation of secondary electrons from that charged portion and the orbit of secondary electrons after the generation are influenced so that not only the contrast of the image is changed but also the image is distorted without reflection of the actual shape of the circuit pattern. This charge state is sensitive to the condition of electron-beam irradiation, so that if the speed or range of irradiation of the electron beam is changed, an image quite different in contrast is obtained even in one and the same position and even in one and the same circuit pattern.
In order to detect a defect being unable to be detected by an optical inspection method with respect to the prior art, a method in which inspection is carried out by irradiating a narrowed electron beam onto a sample substrate at a high speed is disclosed in JP-A-59-160948 and JP-A-5-258703 as a method for performing comparison and inspection by means of an electron-beam image acquired by irradiating an electron beam onto an electrically conductive substrate. In this conventional technique, however, there is no description about a method for adjusting the inspection condition with respect to a material such as an electrically insulating material, or the like. Further, as another conventional technique, a method in which a primary electron beam to be irradiated onto a sample substrate is retarded to thereby make irradiation energy low-accelerated, for example, not higher than 2 keV, in order to observe a substrate having an electrically insulating material is described in JP-A-59-155941. This conventional technique is, however, a method in which an electron beam is continuously irradiated onto a certain local region so that an image is acquired after the charge of the local region becomes stable. Accordingly, this conventional technique is unsuitable for inspecting a wide region at a high speed because a long time is required for acquiring the electron-beam image. Further, even in the case where charge in the local region is stable, it is difficult that another region to be compared is controlled to be in the same charge state. For example, it is difficult to inspect a wide region of a semiconductor wafer, or the like.
In the case where not only a converged electron beam small in electron-beam current is slowly irradiated onto a sample but also a long time is taken for signal detection as shown in the conventional SEM, a signal detected in a detection time per unit pixel is integrated to form an image signal of the unit pixel so that an SN ratio necessary for comparison and inspection is obtained. Because the state of charge changes with the passage of time correspondingly to the irradiation time as described above, the image signal changes during integration so that it is difficult to obtain stable contrast. The present inventors have found that, as a method for inspecting a circuit substrate having such an electrically insulating material, it is effective for obtaining stable contrast to shorten the secondary electron signal detection time to thereby eliminate the contrast fluctuation due to the aforementioned process such as integration, or the like, and to thereby suppress the influence on the change of charge with the passage of time. Further, the present inventors have found that an electron-beam image of contrast due to the secondary electron generation efficiency of the material of the sample by irradiating a large probe sized electron beam in a range of from about 10 nm to about 50 nm onto a sample at a high speed to acquire an image instantaneously is more suitable than an electron-beam image based on contour information of a shape acquired by an electron beam converged to a range of from 5 nm to 10 nm as shown in the SEM of the conventional technique. As described above, a theme of the present invention is not only to acquire an electron-beam image of contrast generated from a material instantaneously by scanning a large probe sized electron beam at a high speed in comparison with the conventional technique but also to secure the SN ratio or resolution in the electron-beam image sufficiently adapted for image comparison and inspection.
In order to radiate an electron beam at a high speed, detect a signal at a high speed and secure the SN ratio and resolution in the electron-beam image as described above, an electron beam having an electron-beam current larger than that generally used in the SEM needs to be irradiated onto a substrate to be inspected as described in the prior art. As described in the prior art, with a large-current and low-accelerated electron beam, it is difficult to obtain an image of high resolution because of the space-charge effect. As a method to solve this problem, there is a method in which a high-accelerated electron beam is retarded just before a sample so that a substantially low-accelerated electron beam is irradiated onto the sample. In order to carry out the deceleration of the primary electron beam, a negative voltage for deceleration is required to be applied to a sample substrate, a sample stage, or the like. When the primary electron beam retarded by the negative voltage is irradiated onto the sample substrate, secondary electrons having energy of the order of tens of mV are generated from a surface of the substrate. Because an electric field generated by the negative voltage for deceleration acts on the secondary electrons to accelerate the secondary electrons to energy of the order of kV, it is difficult to collect the high-speed secondary electrons to a detector. As a method for collecting secondary electrons to a detector, there has been proposed, in the prior art, a method using a deflector (hereinafter referred to as ExB deflector) for offsetting the quantities of deflection caused by the electric field and magnetic field acting on the primary electron beam and for deflecting secondary electrons by superposing the quantities of deflection on each other. In the case where the detector is located in a place away from the orbit of the primary electron beam, however, the secondary electrons need to be deflected largely by the aforementioned ExB deflector in order to collect the secondary electrons to the detector. If the quantity of deflection is selected to be too large, there arises a problem that the secondary electrons collide with a deflection plate per se of the ExB deflector so that the secondary electrons cannot be led to the detector. Further, if the deflection by the ExB deflector is selected to be intensive, there arises a problem that aberration occurs in the primary electron beam so that it is difficult to converge the electron beam on a surface of the sample substrate through an objective lens, or the like.
Further, as described in the prior art, in order to form an electron-beam image at a high speed, a detection means using a semiconductor detector is used as a detection apparatus for detecting a high-frequency secondary electron signal. This prior art means comprises a semiconductor detector reversely biased and high in response speed, a preamplifier for amplifying an analog signal detected by the semiconductor detector, and means for light-transmitting the analog signal amplified by the preamplifier. The aforementioned semiconductor detector and the aforementioned preamplifier are floated to a positive high electric potential. In this conventional method, the analog signal detected by the semiconductor detector is transmitted as it is by the light-transmitting means. This light-transmitting means, however, is constituted by a light-emitting element for converting an electric signal into a light signal, an optical fiber cable, and a light-receiving element for converting a light signal into an electric signal. Accordingly, there arises a problem that noise generated from the light-emitting element and the light-receiving element is added to the original analog signal to lower the SN ratio in the secondary electron signal.